This disclosure relates to a variable-frequency induction motor drive system and, more particularly, to such a system in combination with a submersible water pump.
Deep well, residential service, submersible pumps presently available in the market are driven with two pole, alternating current (ac) induction motors which have been packaged to survive immersion in the well. The stator portion of the motor is encapsulated with an epoxy (or other suitable material) making it impervious to moisture. A substantial length of electric wire must be connected to the motor leads to provide power to the motor when it is operating at depths of up to 400 feet. The installer of the pump system must make this connection, and is interested in providing a waterproof connection to avoid ground leakage paths. Installation instructions stress the importance of waterproofing these connections, and of testing for ground leakage before power is applied to the system. However, it is reasonable to assume that errors will occur, and periodically, power will be applied while ground leakage paths are present.
The conventional pumping systems for residential wells are supplied with power from a branch circuit breaker which does not include a ground fault interrupter (GFI). The power lines are connected through a pressure switch (two pole, single throw) to the two input terminals of a single phase motor. The total wire lengths between the pressure switch and the motor may be as much as 500 feet with a portion of the length buried in the ground, and the remainder submerged in the well. When the system pressure falls below a preset level, the pressure switch closes, connecting each side of the power line to one of the two motor terminals.
The most popular motor in the conventional system is a 3/4 hp single phase induction motor, and is supplied from a single phase, 230 volt, 60 cycle source of power. The power system is center grounded, i.e., the electrical potential of each of the two conductors supplying power to the pressure switch is 115 volts above ground. It is well known that cracks or other imperfections in the motor assembly which allow the ingress of water lead to ground leakage of electrical current, and may ultimately cause a direct ground fault. In addition, any contact between the well water and the conductors bringing power from the well head to the submerged motor will also lead to electrical leakage between the power system and ground. That portion of wiring between the pressure switch and the well head is also susceptible to ground leakage. The three most common sources of ground faults in these systems are 1) loss of insulation integrity in the motor stator, 2) exposure of the power conductors to well water at the connection points, and 3) nicks in the insulation which occur as the pump is being lowered into the well. If the branch breaker in the conventional system includes a GFI function, then the breaker will detect the ground leakage current when the pressure switch closes and will trip, alerting the resident that a ground fault exists. If the branch circuit breaker does not include the GFI function, then the system will continue to operate until the ground leakage reaches a level great enough to trip the breaker from overcurrent. In a conventional system such as this, additional component failure resulting from ground faults in the wiring between the pressure switch and the motor is unlikely.
There are several deficiencies with the present conventional system commercially available including, for example, the following: (1) a four inch pump-motor diameter requires a five inch well casing at a substantial well drilling cost; (2) if a well is pumped dry, the pump may be damaged because the bearings are lubricated and cooled by the water, and the lack of water can lead to bearing failure unless a flow restrictor is added to the waterline at the well head to prevent the output flow from exceeding the well recovery rate; (3) sand, stone chips or other debris in the well may cause the pump to seize or bind leading to a stalled motor condition that may cause motor overheating and damage; (4) if the line voltage is low, the motor is forced to operate at less than the rated magnetic flux, thus requiring more current to produce the same torque leading to motor overheating and the possibility of eventual failure; (5) starting the motor by connecting across the AC power line results in a significant surge in input current with each start and frequent restarts, such as those experienced when power is frequently interrupted during thunder storms, can also lead to motor overheating and failure; and (6) because of the heating penalty associated with each start, the hysteresis in the pressure switch must be increased to a value great enough to ensure that the motor doesn't restart too frequently forcing the homeowner to endure pressure variations of a much as 50% between pump starts.
An improved pumping system which addresses these deficiencies is disclosed in U.S. patent application Ser. No. 08/723,202, now U.S. Pat. No. 5,883,489, entitled "High Speed Deep Well Pump for Residential Use" This system employs a solid-state power unit for converting 230 volt, single phase, 60 Hz power to 230 volt, three phase, variable frequency power allowing the pump motor speed to be controlled between zero and 10,000 revolutions per minute. Such a pumping system can overcome the deficiencies pointed out above while providing a superior pressure regulated water supply where variations in pressure are restricted to much lower levels, and intelligence implicit in the microcontroller software can detect the existence of control problems, take appropriate protective measure, and alert the homeowner of the problem. However, the existence of a ground fault condition poses particular problems for such a system.
FIG. 2 is a schematic representation of the power circuit disclosed in the aforementioned U.S. patent application Ser. No. 08/723,202, now U.S. Pat. No. 5,883,489, for supplying power to a submersible pump in a residential water system. The power source is a 230 volt, single phase, center grounded, 50/60 Hz source which is connected to a bridge rectifier comprised of diodes 16, 18, 20, and 22. This rectifier converts the 50/60 Hz source to full-wave rectified power. The output of the bridge rectifier connects to positive and negative buses 24, 25. A capacitor 26 is connected between the two buses to smooth the power minimizing the effects of the ripple voltage inherent in the full wave rectified power source. A three phase bridge arrangement 27 of power switching devices 28, 30, 32, 34, 36 and 38, which are typically either Insulated Gate Bipolar Transistors (IGBT) or Field Effect Transistors (FET) are used to convert the filtered output of the bridge rectifier into a source of variable voltage, variable frequency power to control the speed and torque of the induction motor in a manner well known in the art. The output of the three phase bridge 27 is a balanced three phase voltage which may be either a sine wave or a square wave where the volt-seconds of each cycle is maintained constant through the well known principle of pulse width modulation (PWM). It should be noted that a power diode is connected in an inverse parallel arrangement across each of the power switches in the three phase bridge to provide a path for reverse current flow through each switch position, and is not shown in FIG. 2 for simplicity. A current sensing resistor 29 is connected in the negative bus between the filter capacitor 26 and the three phase bridge to provide a controller 42 with the signals necessary for protection and regulation functions.
The three phase bridge switching devices (IGBT/FET) have a rapid turn-on characteristic, typically between 15-100 nanoseconds, and the result is a rapidly rising current wave front through the current sensing resistor 29 and due to the parasitic inductance of this resistor, a voltage spike appears across the resistor 29 during the switching interval. The current regulating system interprets voltage across the sense resistor 29 as an analog of current, and the voltage spike is erroneously interpreted as a rapidly rising overcurrent leading to an incorrect response by the current protection system. A filter comprised of resistors 84 and 86, and capacitor 87 must be added to the system to prevent the overcurrent protection function from invoking an incorrect response. The selection of component values for this filter are a design tradeoff between maintaining rapid overcurrent protection, and avoiding nuisance overcurrent trips. However, the presence of the filter always leads to a delay in the current sense signal making it difficult to provide the speed necessary to protect the switching devices 28-38 should a short circuit occur in the output circuitry.
The phantom line 39 indicates a ground in one of the motor leads for illustrative purposes. This ground may be the result of nicked insulation, poorly insulated motor terminal connections, or a motor stator leakage to ground. It may also be a high impedance leakage path, or a solid low impedance path. Assuming that a short to ground exists prior to motor start up, and further that the input power line is of the indicated polarity, then the gating of a positive power device, such as 32, will result in a short circuit from the positive power terminal 12 through diode 16, power switch 32, through the leakage path 39, through the power circuit ground and returned to the negative side of the power source. This results in 115 volts(ac) being applied to the circuit described. Note in particular that this path does not include the current sensor 29 and hence the control would have no way of knowing that the short circuit had occurred, and the failure of one or more positive side power switches will likely result long before any protective functions can be activated.
Assuming, however, that the first power switch to be gated is a negative side cell such as 38, the short circuit loop initiated when switch 38 is gated is from voltage source lead 14 through the system ground, through the leakage path 39, through switch 38, current sensor 29, and diode 22. This path includes the current sensor 29 and the fault current can theoretically be detected. However, if the short circuit is a very low impedance, the current in the loop will rise so rapidly that the delays in the filter described above will allow the current to rise to levels adequate to destroy the power switch 38 before the current protection function has time to work. Thus, it can be seen that gating of a positive side switch will result in its destruction because the current path does not include a current sensor, and the gating of a negative side cell may result in its destruction because of delays introduced into the protection function to avoid nuisance trips or false shutdowns. Accordingly, it is desirable to provide a method for detecting ground leakage paths prior to applying full power to the three phase bridge.